1. Field of the Invention
The invention relates in general to servomechanisms for use with magnetic tape recorders; more specifically, the invention provides a playback servomechanism for so controlling the tape driving capstan that the recorded tracks will precisely line up with the path taken by the moving heads of a rotating-head magnetic tape recorder.
2. DESCRIPTION RELATIVE TO THE PRIOR ART
With reference to the tape recording art, tracking is the process of keeping a playback head on the path of a track already recorded on magnetic tape. Put another way, it is a measure of the ability of the head scan correctly the recorded track when in the playback mode. Since tracking basically refers to a positional adjustment, presently available magnetic tape players influence the tape scan by servo controlling the position of either the tape or the head with respect to each other.
Although tracking is a problem endemic to all tape players, it attains a particular significance in the case of rotary-head players where the playback head must follow one-by-one a plurality of track segments recorded across the width of the tape while the tape itself is advancing. The usual commercial, portable rotary-head tape recorder intended for use in home or business employs a helical-scan rotary head. In such a recorder, tracking control is provided by adjusting the speed of the rotating heads. The control process generally begins by providing a control track during the record mode. For example, if a video signal is being recorded, as each vertical sync pulse occurs, the video record head is rotated at such a speed that it has a predetermined location with respect to the tape. At this time, a pulse is recorded along the edge of the tape by a control track head. If the tape transport is running at an even speed, these pulses will be evenly spaced along the tape. Upon playback, the control track head -- positioned a fixed distance from the playback head rotational axis -- senses the control track and produces a pulse which is processed by the servo electronics and applied to a head servo to correct for transport errors.
Servos for head control in the playback mode generally use two signals: one for reference as to where the head should be in its rotational movement and the other for feedback as to where the head is actually located. The incoming pulse from the control track is used as the reference while a pulse from a head position sensor forms the feedback. The latter may, for example, be provided by a small magnet mounted inside the head drum on its rotating mechanism. A small pickup head detects its passage for each revolution of the head and generates a head position pulse. The degree to which the control track pulse coincides with the head position pulse determines the actual position of the playback head relative to the recorded track; appropriate correction is then rendered possible.
Since the pulses of the control track were recorded so as to coincide with a definite position of the record head, the physical distance of the control track head along the tape from the point where the video head began recording a given track is of critical importance. If this distance differs on playback from that obtained during record, the playback head -- though properly adjusted insofar as the servomechanism is concerned -- will not correctly follow the recorded track. It is as if the playback head follows a path in which the electronics expects the head to overlie the track -- but in fact it does not. Similarly, a stretched tape or a control track head slightly out of alignment will cause a minor mistiming in the arrival of the control pulses to the servo, and a similar mistracking effect will result. Customarily, a tracking control is provided for delaying the control pulse electronically before it is compared to the feedback signal. This has the effect of moving the control head sideways until it correctly scans the control track. In actual use, the delay is manually varied until an acceptable television picture results.
Servos for capstan control, and particularly capstan servos in combination with head wheel servos, were first widely used in broadcast quad-head video recorders. Because of its complex aspects, including its servomechanisms, the quad-head recorder is far too large and complex for any use outside of the broadcast industry. With the advent of non-broadcast video tape usage, the smaller, simpler, and cheaper helical video recorder was preferred. To pare down the cost and size of a video recorder, a number of compromises had to be taken -- including the simplification of the servomechanisms. Therefore, the practice of using the head servo for correcting playback tracking errors was economically logical since a head servo must be provided anyway during recording if a definite recording pattern -- i.e., the location of the vertical sync -- is to be established on the tape. Furthermore, if the output of the playback heads is "switches," a head servo is frequently used for coinciding the moment of playback switching with that obtained on recording.
Resort to a capstan servo is usually required, even for portable helical video recorders, if one of the recording features includes editing of a video signal. In that situation, the previously recorded video on the tape must be aligned accurately with a new incoming video. To do this, the speed of the tape must be varied until the vertical sync pulses of both signals align, and then the tape must be held at a constant speed to maintain this alignment. This ordinarily means that the capstan must be servo controlled.
The foregoing methods of servo control have been adapted in the various forms of commercially available helical recorders. Having said this, there remains proposals for other types of tracking control servos. One such proposal is seen in Japanese Pat. No. 46-22502, issued June 26, 1971. This patent discloses an apparatus for detecting longitudinal slip of a helical playback head as it begins to cross over and overlap two tracks. The video signal at that moment will include the desired horizontal sync from the playback video as well as an undesired horizontal sync from an adjacent track. The double sync thus produced when the playback head incorrectly overlaps two tracks is processed so as to provide a signal indicative of the direction of tracking error. The capstan speed is then adjusted to eliminate the track overlap and thereby to resume correct tracking. Known in the servo control art as a "bang, bang" type of control, the approach described in the Japanese patent requires an appreciable tracking error to produce a second sync signal in the playback signal that can be detected and used to change the tracking. No additional correction is available until the tracking is again in error sufficiently to produce another correction. Thus, besides being an intermittent control, such a tracking servo provides an imprecise control as well, since it depends on a gross degree of misalignment in order to generate a control signal.
Mistracking is a playback phenomenon; however, capstan servos are sometimes used during recording for ensuring that the tracks are laid down in some predetermined pattern. For example, U.S. Pat. No. 3,549,797 discloses a servo system that adjusts the tape speed for maintaining the horizontal sync on adjacent tracks in side-by-side alignment. Intended for record only, the system includes a pair of stationary pickup heads disposed toward opposite edges of the tape for scanning the tape in a longitudinal direction, crossing over the slant-tracks and sensing the incidence of horizontal sync pulses as they do. The incidence of horizontal sync pulses from opposite edges provide an indication of the alignment of sync pulses in adjacent tracks. When appropriate adjustment of the capstan is made, the tracks should mutually align so that corresponding image lines appear in adjacent slant-tracks. It is therefore hoped that the format of the tape will be more tolerant to crossover by the playback heads since, at least, corresponding elements of horizontal image lines would be the ones sensed by the improperly tracking head.
One basic problem with present systems of tracking control lies in the separation of the playback head from the point at which the tracking control signal is sensed. Being responsive to any condition impressed upon the tape, the control head not only replicates recording conditions but inseparably includes playback anomalies due to stretch, skew, and alignment errors introduced by the playback machine along the span of tape between the playback head and the control track head. Efforts to eliminate this problem may be simply to reduce the span of tape affected until the control point sensed is near, or at, the playback head itself. The Japanese patent hereinbefore mentioned is characteristic of this approach. However, since in that case the control information is sensed by the video playback head itself, it can only respond with an intermittent, "bang-bang" type of control.